Introduction:
HER2 expression is a predictor of poor survival in patients with head and neck squamous cell carcinoma (HNSCC) and is associated with resistance to cetuximab therapy. There is a wide range of HER2 expression reported in head and neck cancer – higher prevalence in salivary cancers and lower prevalence in squamous cell carcinoma. Because the anti-HER2 antibody, trastuzumab, is effective only when cells are dependent on HER2 signaling for survival, it has largely failed in clinical trials for HNSCC.  However, the recent success of antibody drug conjugates (ADCs; antibody linked to a cytotoxic payload) in HER2-positive breast and gastric cancer suggests that other solid tumors may benefit from this approach, even in tumors with low HER2 expression. We assessed the in vitro and in vivo efficacy of T-DM1, a bioconjugate of a microtubule inhibitor, emtansine, linked to an anti-HER2 antibody, trastuzumab, in HNSCC.

MATERIALS AND METHODS: HER2 expression was evaluated in HNSCC cell lines using western blotting, and relative expression was measured based on HER2 expression levels in the positive control cell line, BT-474. In vitro cytotoxicity of T-DM1 in HNSCC was measured using the MTS assay, and IC50 was calculated. A flank tumor xenograft model with UM-SCC-47 (low HER2 expression; 3.6%) in nu/nu mice was used to evaluate in vivo efficacy of T-DM1. Tumor-bearing mice were treated with four weekly 15mg/kg intraperitoneal doses of T-DM1, trastuzumab, and saline. The fourth dose was administered using fluorescently conjugated drugs to assess drug distribution using near-infrared imaging. Tumor volume (length * width² * 0.5) was measured throughout the treatment period. Missing tumor volume data due to early sacrifice of mice with tumor volume greater than 750mm³ was imputed using generalized additive models. Tumor volume over time among the treatment groups were compared using linear mixed-effects models.

RESULTS: HER2 expression in HNSCC cell lines was between 1.5% - 10.9% relative to positive control and correlated with in vitro cytotoxicity after T-DM1 treatment with two- to four-fold sensitivity compared to negative control. After four weekly doses of the three treatments in an in vivo mouse xenograft model, there was statistically significant difference in the tumor volume curves among the treatment groups, where tumors treated with T-DM1 showed slower rate of tumor volume increase compared to that of trastuzumab and saline (Figure 1; p = 0.01). In vivo near-infrared imaging of mice after the fourth dose of the fluorescently conjugated drugs showed high fluorescence intensity in the tumor (signal to background ratio: 2.7 + 0.3, 3.2 + 0.5, T-DM1 and trastuzumab, respectively).

CONCLUSION: Although HER2 expression is low in HNSCC cell lines complicating previous trials of HER2-targeted drugs in HNSCC, in vitro and in vivo studies show potential for targeting HER2 in patients with HNSCC using ADCs, a novel class of therapeutics.

Figure 1. Tumor volume after 4 injections (red arrows) of T-DM1, trastuzumab, and saline in a HNSCC xenograft mice model (UM SCC 47).